Caterpillars use the substrate as their external skeleton

Abstract

Animals that lack rigid structures often employ pressurization to maintain body form and posture. Structural stability is then provided by incompressible fluids or tissues and the inflated morphology is called a hydrostatic skeleton. However, new ground reaction force data from the caterpillar, Manduca sexta suggest an alternate strategy for large soft animals moving in complex three dimensional structures. When crawling, Manduca can keep its body primarily in tension and transmit compressive deformation forces through the substrate. This effectively allows the caterpillar to minimize reliance on a hydrostatic skeleton and helps it conform to the environment. We call this alternative strategy an "environmental skeleton".

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Figures and Tables

Figure 1 Two dominant modes of locomotion in caterpillars: crawling and inching. Some caterpillars with selective proleg reduction develop unique intermediate inch-crawl gaits which will be reported later in a full research article.

Figure 2 A simple scenario illustrates the difference between using hydrostatic skeleton and environmental skeleton for posture control. (A) A doubly supported soft beam will buckle dramatically under the influence of gravity. (B) Pressurization is one way to increase stiffness and therefore maintaining the linear configuration. The buckling side (dorsal) will experience greater tensile stress according to the beam theory. (C) Alternatively, and overall increase of axial tension can straighten the body without pressurization. Yet the substrate needs to withstand substantial compressive load from the animal without buckling.

Figure 3 Substrate stiffness greatly affects the body control of Manduca caterpillars as demonstrated in the suspension experiment. (A) There are no observable kinematics differences when the caterpillar is walking a stiff substrate. (B) Flexible substrate has variable curvature which the caterpillar simply adapts to it. (C) Soft substrate that cannot support bending load nor compression leads to gait confusion in the caterpillar's normal locomotion since the substrate fails to maintain the axial tension in the caterpillar body.

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